Object recognition with 3D models

ABSTRACT

An “active learning” method trains a compact classifier for view-based object recognition. The method actively generates its own training data. Specifically, the generation of synthetic training images is controlled within an iterative training process. Valuable and/or informative object views are found in a low-dimensional rendering space and then added iteratively to the training set. In each iteration, new views are generated. A sparse training set is iteratively generated by searching for local minima of a classifier&#39;s output in a low-dimensional space of rendering parameters. An initial training set is generated. The classifier is trained using the training set. Local minima are found of the classifier&#39;s output in the low-dimensional rendering space. Images are rendered at the local minima. The newly-rendered images are added to the training set. The procedure is repeated so that the classifier is retrained using the modified training set.

RELATED APPLICATION

This application claims priority from U.S. provisional application No.61/222,245, filed Jul. 1, 2009, entitled “Object Recognition with 3DModels”, which is incorporated by reference herein in its entirety.

FIELD OF THE INVENTION

The present invention relates to machine vision, including objectrecognition. More particularly, the present invention relates totraining classifiers to perform view-based object recognition.

BACKGROUND OF THE INVENTION

Over the past few years, the object recognition community has taken onthe challenge of developing systems that learn to recognize hundreds ofobject classes from a few examples per class. The standard data setsused for benchmarking these systems contained, in average, fewer thantwo hundred images per class (as opposed to sets of thousands ofmeticulously segmented object images that were used in earlier work onobject detection). Benchmarking on such small data sets is inherentlyproblematic. The test results cannot be generalized and can bemisleading.

There have been efforts of building larger databases of manuallyannotated, natural images. However, the somewhat arbitrary selection ofimages and the missing ground truth make it difficult to systematicallyanalyze specific properties of object recognition systems, such asinvariance to pose, scale, position, and illumination. A database forshape-based object recognition which addresses these issues is NORB fromthe Courant Institute at New York University. Pictures of objects weretaken with consideration of viewpoint and illumination. The images weresynthetically altered to add more image variations, such as objectrotation, background, and “distractors”.

Taking the idea of controlling the image generation one step furthertakes us to fully synthetic images rendered from realisticthree-dimensional (3D) computer graphics models. Some view-based objectrecognition systems have been trained and evaluated on synthetic images.At least one face recognition system and one object recognition systemhave been trained on views of 3D models and tested on real images. 3Dmodels have also been used in a generative approach to objectrecognition in which rendering parameter values are optimized such thatthe synthetic image best matches a given photographic image. To avoidgetting trapped in local minima, this analysis-through-synthesisapproach requires a good initial estimate of the rendering parametervalues, making it unsuited to many object recognition/detection tasks.

SUMMARY OF THE INVENTION

A view-based object recognition system uses a model of an object'sappearance in order to determine whether the object is present in agiven image. The view-based object recognition system generates(“learns”) the model when the system is trained using a training dataset. The training data set includes images that are positive examples(i.e., the target object is present) and images that are negativeexamples (i.e., the target object is absent). Each training image islabeled correctly regarding whether it is a positive example or anegative example.

Like any other image, a training image can be either natural orsynthetic. A natural image faithfully represents the appearance of areal-world object and is generated by, for example, taking a picture ofthe object using a camera or other image sensor. A synthetic image isany image other than a natural image. For example, a computer-aideddesign (CAD) model is rendered to generate the synthetic image. Traininga view-based object recognition system using synthetic images (asopposed to natural images) has several advantages.

An “active learning” method is presented, in which the generation ofsynthetic training images is controlled within an iterative trainingprocess. (The term “active learning” is used herein to refer to alearning method that actively generates its own training data.) Theprimary idea of active learning is to find valuable and/or informativeobject views in a low-dimensional rendering space and then add theseviews iteratively to the training set. In each iteration, instead of“bootstrapping” the classifier (by adding particular sample images froma given database of images), new views are generated. A sparse trainingset is iteratively generated by searching for local minima of aclassifier's output in a low-dimensional space of rendering parameters.

In one embodiment, an active learning method trains a compact classifierfor view-based object recognition. An initial training set is generated.The classifier is trained using the training set. Local minima are foundof the classifier's output in the low-dimensional rendering space.Images are rendered at the local minima. The newly-rendered images areadded to the training set. The procedure is repeated so that theclassifier is retrained using the modified training set.

The features and advantages described in the specification are not allinclusive and, in particular, many additional features and advantageswill be apparent to one of ordinary skill in the art in view of thedrawings, specification, and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The patent or application file contains at least one drawing executed incolor. Copies of this patent or patent application publication withcolor drawings will be provided by the U.S. Patent and Trademark Officeupon request and payment of the necessary fee.

FIG. 1 is a flowchart of an active learning method that trains a compactclassifier for view-based object recognition, according to oneembodiment of the invention.

FIG. 2 shows five computer graphics models, according to one embodimentof the invention.

FIG. 3 shows photographs of the printed versions of the five models ofFIG. 2, according to one embodiment of the invention.

FIG. 4 is a block diagram of a system for performing the active learningmethod of FIG. 1, according to one embodiment of the invention.

FIG. 5 shows examples of rendered images after scaling and smoothing,according to one embodiment of the invention.

FIG. 6 shows examples of initially-selected views and views at nearbylocal minima, according to one embodiment of the invention.

The figures depict various embodiments of the present invention forpurposes of illustration only. One skilled in the art will readilyrecognize from the following discussion that alternative embodiments ofthe structures and methods illustrated herein may be employed withoutdeparting from the principles of the invention described herein.

DETAILED DESCRIPTION OF THE INVENTION

Various embodiments are now described with reference to the figureswhere like reference numbers indicate identical or functionally similarelements. Also in the figures, the left-most digit of each referencenumber corresponds to the figure in which the reference number is firstused.

Reference in the specification to “one embodiment” or to “an embodiment”means that a particular feature, structure, or characteristic describedin connection with the embodiments is included in at least oneembodiment of the invention. The appearances of the phrase “in oneembodiment” or “an embodiment” in various places in the specificationare not necessarily all referring to the same embodiment.

Some portions of the detailed description that follows are presented interms of algorithms and symbolic representations of operations on databits within a computer memory. These algorithmic descriptions andrepresentations are the means used by those skilled in the dataprocessing arts to most effectively convey the substance of their workto others skilled in the art. An algorithm is here, and generally,conceived to be a self-consistent sequence of steps (instructions)leading to a desired result. The steps are those requiring physicalmanipulations of physical quantities. Usually, though not necessarily,these quantities take the form of electrical, magnetic, or opticalsignals capable of being stored, transferred, combined, compared, andotherwise manipulated. It is convenient at times, principally forreasons of common usage, to refer to these signals as bits, values,elements, symbols, characters, terms, numbers, or the like. Furthermore,it is also convenient at times to refer to certain arrangements of stepsrequiring physical manipulations or transformation of physicalquantities or representations of physical quantities as modules or codedevices, without loss of generality.

However, all of these and similar terms are to be associated with theappropriate physical quantities and are merely convenient labels appliedto these quantities. Unless specifically stated otherwise as apparentfrom the following discussion, it is appreciated that throughout thedescription, discussions utilizing terms such as “processing” or“computing” or “calculating” or “determining” or “displaying” or thelike refer to the action and processes of a computer system, or similarelectronic computing device (such as a specific computing machine), thatmanipulates and transforms data represented as physical (electronic)quantities within the computer system memories or registers or othersuch information storage, transmission, or display devices.

Certain aspects of the present invention include process steps andinstructions described herein in the form of an algorithm. It should benoted that the process steps and instructions of the present inventioncould be embodied in software, firmware, or hardware and, when embodiedin software, could be downloaded to reside on and be operated fromdifferent platforms used by a variety of operating systems. Theinvention can also be in a computer program product that can be executedon a computing system.

The present invention also relates to an apparatus for performing theoperations herein. This apparatus may be specially constructed for thepurposes, e.g., a specific computer, or it may comprise ageneral-purpose computer selectively activated or reconfigured by acomputer program stored in the computer. Such a computer program may bestored in a computer readable storage medium such as, but not limitedto, any type of disk including floppy disks, optical disks, CD-ROMs,magnetic-optical disks, read-only memories (ROMs), random accessmemories (RAMs), EPROMs, EEPROMs, magnetic or optical cards, applicationspecific integrated circuits (ASICs), or any type of media suitable forstoring electronic instructions, and each coupled to a computer systembus. Memory can include any of the above and/or other devices that canstore information/data/programs. Furthermore, the computers referred toin the specification may include a single processor or may bearchitectures employing multiple processor designs for increasedcomputing capability.

The algorithms and displays presented herein are not inherently relatedto any particular computer or other apparatus. Various general-purposesystems may also be used with programs in accordance with the teachingsherein, or it may prove convenient to construct more specializedapparatus to perform the method steps. The structure for a variety ofthese systems will appear from the description below. In addition, thepresent invention is not described with reference to any particularprogramming language. It will be appreciated that a variety ofprogramming languages may be used to implement the teachings of thepresent invention as described herein, and any references below tospecific languages are provided for disclosure of enablement and bestmode of the present invention.

In addition, the language used in the specification has been principallyselected for readability and instructional purposes and may not havebeen selected to delineate or circumscribe the inventive subject matter.Accordingly, the disclosure of the present invention is intended to beillustrative, but not limiting, of the scope of the invention, which isset forth in the claims.

While particular embodiments and applications of the present inventionhave been illustrated and described herein, it is to be understood thatthe invention is not limited to the precise construction and componentsdisclosed herein and that various modifications, changes, and variationsmay be made in the arrangement, operation, and details of the methodsand apparatuses of the present invention without departing from thespirit and scope of the invention as it is defined in the appendedclaims.

A view-based object recognition system uses a model of an object'sappearance in order to determine whether the object is present in agiven image. The model represents the object's appearance in an imageunder various conditions (e.g., different poses and/or illuminations).The view-based object recognition system generates (“learns”) the modelwhen the system is trained using a training data set. The training dataset includes images that are positive examples (i.e., the target objectis present) and images that are negative examples (i.e., the targetobject is absent). Each training image is labeled correctly regardingwhether it is a positive example or a negative example.

Like any other image, a training image can be either natural orsynthetic. A natural image faithfully represents the appearance of areal-world object and is generated by, for example, taking a picture ofthe object using a camera or other image sensor. A synthetic image isany image other than a natural image. One type of synthetic image isgenerated based on a natural image. For example, the natural image isdistorted or transformed in some way to generate the synthetic image.Another type of synthetic image is generated independently of a naturalimage. For example, a computer-aided design (CAD) model is rendered togenerate the synthetic image. In order to distinguish between these twotypes of synthetic images, the first type will be referred to aspartially-synthetic (since it was generated based on a natural image),and the second type will be referred to as fully-synthetic (since it wasgenerated independently of a natural image).

Training a view-based object recognition system using synthetic images(as opposed to natural images) has several advantages. First of all,large numbers of training images can be generated automatically. Also,full control is available over image generation parameters, includinginternal and external camera parameters, illumination, composition ofthe scene, and animation of the scene. Finally, ground truth is knownfor the location, scale, and orientation of each object. In videosequences, the frame rate, the camera motion, and the motion of objectsare known.

Supervised machine learning methods, such as statistical classifiers,have been used to address the problem of view-based object recognition.Consider training a view-based classifier using fully-synthetic trainingimages (e.g., images generated based on three-dimensional (3D) models).The training will likely face two problems. First, the training set willbe very large, which can break the learning algorithm. Second, thesolutions are not sparse, so the classifiers will be slow at run-time.One of the largest groups of modern machine learning algorithms is thegroup of kernel-based machine learning algorithms, which includessupport vector machines (amongst others). It is possible to compute aquadratic matrix of size N×N where N is the number of samples that can“break” a kernel-based machine learning algorithm (see O. Chapelle,“Training a Support Vector Machine in the Primal”, Neural Computation,Vol. 19, No. 5, pp. 1155-1178, March 2007). A large number of sampleswill also make training slow, since the time complexity in kernelalgorithms is polynomial in the number of samples. The complexity atruntime is also affected for support vector machines (and possibly forother kernel-based algorithms as well). The number of training datapoints that are used in computing the classification result increaseslinearly with the size of the training set. Basically, runtimecomplexity during classification is linear in training sample size. Whatis needed is a technique that can train (e.g., build or compute) compactclassifiers from sparse data sets.

An “active learning” method is now presented, in which the generation ofsynthetic training images is controlled within an iterative trainingprocess. (Note that the term “active learning” in the machine learningcommunity refers to learning from data with hidden labels. The term“active learning” is used herein to refer to something different—namely,a learning method that actively generates its own training data.) Theprimary idea of active learning is to find valuable and/or informativeobject views in a low-dimensional rendering space and then add theseviews iteratively to the training set. In each iteration, instead of“bootstrapping” the classifier (by adding particular sample images froma given database of images), new views are generated. A sparse trainingset is iteratively generated by searching for local minima of aclassifier's output in a low-dimensional space of rendering parameters.

FIG. 1 is a flowchart of an active learning method that trains a compactclassifier for view-based object recognition, according to oneembodiment of the invention. In step 110, an initial training set isgenerated. In step 120, the classifier is trained using the trainingset. In step 130, local minima are found of the classifier's output inthe low-dimensional rendering space. In step 140, images are rendered atthe local minima. In step 150, the newly-rendered images are added tothe training set. In step 160, the procedure is repeated starting fromstep 120, so that the classifier is retrained using the modifiedtraining set.

Steps 110-160 are further explained below in conjunction with an exampleof a classification task and a system for performing the active learningmethod of FIG. 1. In this example, object recognition is performed forfive classes of objects. Each object is represented by a texture-less 3Dmodel with 30,000 surface triangles. FIG. 2 shows the five computergraphics models, according to one embodiment of the invention. In FIG.2, the illustrated models represent (from left to right) a bust of thegoddess Aphrodite, a bear lying down, an elephant standing up, a horsestanding up, and a shoe sole. Each of the five models is also printed ona 3D printer in order to be able to test the object recognition systemon natural images. FIG. 3 shows photographs of the five printed models,according to one embodiment of the invention.

In this example, to generate an image (either synthetic or natural), acamera viewpoint (either virtual or actual) is moved on a sphere aroundthe model, pointing towards the model's center. The model is illuminatedby a point light source (which can be positioned on a sphere around themodel) and by ambient light. This image generation setup results in alow-dimensional rendering space (specifically, a six-dimensionalrendering space). The six rendering parameters are: 1) the viewpoint'slocation in azimuth, 2) the viewpoint's location in elevation, 3) theviewpoint's rotation around its optical axis, 4) the point lightsource's location in azimuth, 5) the point light source's location inelevation, and 6) the intensity ratio between the ambient light and thepoint light source.

FIG. 4 is a block diagram of a system for performing the active learningmethod of FIG. 1, according to one embodiment of the invention. Thesystem 400 is able to train a compact classifier for view-based objectrecognition. The illustrated system 400 includes an active learningmodule 405, a renderer 410, a classifier trainer 415, a classifiertester 420, storage 425, and operating system tools 430.

In one embodiment, the active learning module 405 (and its componentmodules), the renderer 410, the classifier trainer 415, the classifiertester 420, and the operating system tools 430 are one or more computerprogram modules stored on one or more computer readable storage mediumsand executing on one or more processors. The storage 425 (and itscontents) is stored on one or more computer readable storage mediums.Additionally, the active learning module 405 (and its componentmodules), the renderer 410, the classifier trainer 415, the classifiertester 420, the operating system tools 430, and the storage 425 arecommunicatively coupled to one another to at least the extent that datacan be passed between them. In one embodiment, the operating systemtools 430 are executing as one or more parts of an operating system on apersonal computer, and the active learning module 405, the renderer 410,the classifier trainer 415, and the classifier tester 420 are executingon the same personal computer.

The illustrated storage 425 stores a classifier 445, a training set 450,and one or more 3D models 460. The classifier 445 is a statisticalclassifier such as a support vector machine (SVM) or a nearest-neighborclassifier. In one embodiment, the classifier 445 is a SVM with aGaussian kernel. The SVM parameter values are optimized in initialexperiments and then kept fixed throughout (e.g., s=2:0 and C=10, wheres represents the kernel width and C represents the error cost). SVMs arefurther described in V. Vapnik, Statistical Learning Theory,Wiley-Interscience, 1998. Implementations of SVMs include, for example,the LIBSVM library (C.-C. Chang et al., “LIBSVM: a Library for SupportVector Machines”, 2001).

The classifier 445 operates using a vector of image feature values(“feature vector”). Any image feature can be used, such as gray-values,normalized gray-values, or histograms of gradients. In one embodiment,orientation histograms or histograms of gradients are used for thefeature vector. The histograms are computed at five fixed locationswithin a 23×23 image, resulting in a 640-dimensional feature vector thatis normalized to unit length. The 128-dimensional histograms arecomputed at the following (x/y) locations: (9/9), (15/9), (12/12),(9/15), and (15/15).

The classifier 445 can be trained using the classifier trainer 415 andtested using the classifier tester 420.

The training set 450 is a set of images that are used to train theclassifier 445 using the classifier trainer 415. An image in thetraining set 450 is a synthetic image that was generated by using therenderer 410 to render a 3D model 460.

A 3D model 460 is a model of a three-dimensional object and can berendered using the renderer 410 to produce a synthetic image.

The operating system tools 430 include a random number generator 455.The random number generator 455 can generate a random (or pseudo-random)number.

The renderer 410 is a conventional software application that can rendera 3D model 460, such as Blender (an open source software package for 3Dmodeling and rendering). The renderer 410 renders a model 460 based on aset of rendering parameter values. In the example introduced above,there are six rendering parameters (the viewpoint's location in azimuthand elevation, the viewpoint's rotation around its optical axis, thepoint light source's location in azimuth and elevation, and theintensity ratio between the ambient light and the point light source).

In one embodiment, the renderer 410 renders a 3D model 460 at aresolution of 100×100 pixels. The renderer 410 also scales and smoothesthe rendered (synthetic) image, resulting in a 23×23 pixel gray-value(synthetic) image. FIG. 5 shows examples of the rendered (synthetic)images after scaling and smoothing, according to one embodiment of theinvention.

The classifier trainer 415 is a conventional software application thatcan train a classifier 445 given a training set 450 (e.g., a set oftraining images).

The classifier tester 420 is a conventional software application thatcan test a classifier 445 to determine the classifier's classificationaccuracy. In one embodiment, the classifier 445 is tested using 40,000views per class, where each view is randomly drawn from thelow-dimensional space of rendering parameters. The classifier's outputis computed, using the real-valued output of the SVM. (For samples ofthe negative class (label “−1”), the output is multiplied by −1.) Themost difficult views from each class are determined (e.g., the 100 viewswith the lowest accuracy rates).

The active learning module 405 includes a control module 435 and a localminima finder 440. The control module 435 controls the operation of theactive learning module 405 so that the active learning module 405 cantrain a compact classifier 445 for view-based object recognition. Thecontrol module 435 is further described below with reference to FIG. 1.

The local minima finder 440 finds local minima (and the associatedrendering parameter values) of a classifier's output in alow-dimensional rendering space. A set of views (e.g., the 100 mostdifficult views from each class) are used as starting points of anoptimization algorithm (e.g., the Nelder-Mead simplex algorithm). Notethat the values of the six rendering parameters are already known forevery view. A number of iterations of the algorithm (e.g., ten) arecomputed in order to find local minima (and the associated renderingparameter values) of the classifier's output in the rendering space. Theclassifier's output is computed, using the real-valued output of theSVM. (For samples of the negative class (label “−1”), the output ismultiplied by −1.) FIG. 6 shows examples of the initially-selected(i.e., starting point) views and the views at the nearby local minima,according to one embodiment of the invention. In FIG. 6, example pairsare vertically arranged with the initial views on top and the views atnearby local minima on the bottom.

In general, active learning achieves the same error rate (e.g., equalerror rate or “EER”) with significantly smaller training sets andsignificantly fewer support vectors than training on a random selectionof object views. In other words, active learning trains a “compact”classifier.

Returning to FIG. 1, in step 110, an initial training set 450 isgenerated, where the initial training set 450 includes one or moresynthetic images. Each synthetic image is generated by the renderer 410based on a 3D model 460 and a set of rendering parameter values. In oneembodiment, the initial training set 450 is comprised of randomlyselected samples from the low-dimensional space of rendering parameters.

In one embodiment, the control module 435 performs step 110. Forexample, the control module 435 uses the random number generator 455 toobtain one randomly-selected value for each of the rendering parametervalues. (In the example introduced above, there are six renderingparameters, so each randomly-selected sample has one randomly-selectedvalue for each of these six parameters.) The control module 435 thenuses the renderer 410 to generate a synthetic image of a 3D model 460based on the rendering parameter values. The synthetic image is thenadded to the initial training set 450. In one embodiment, the initialtraining set 450 includes 200 samples per class (e.g., 200 syntheticimages per 3D model 460).

In step 120, the classifier 445 is trained using the training set 450.In one embodiment, the control module 435 performs step 120. Forexample, the control module 435 uses the classifier trainer 415 to trainthe classifier 445 using the training set 450. The first time that step120 is executed, the training set 450 is the initial training set. Thesecond and subsequent times that step 120 is executed, the training set450 will have been modified, as explained below in conjunction with step150.

In step 130, local minima are found of the classifier's output in thelow-dimensional rendering space. In one embodiment, the control module435 performs step 130. For example, the control module 435 first usesthe classifier tester 420 to test the classifier 445 and determine theclassifier's classification accuracy. The classifier tester 420determines the most difficult views from each class (e.g., the 100 viewswith the lowest accuracy rates). The control module 435 then uses thelocal minima finder 440 to find local minima (and the associatedrendering parameter values) of the classifier's output, using the mostdifficult views as starting points.

In step 140, images are rendered at the local minima (using theassociated rendering parameter values). In one embodiment, the controlmodule 435 performs step 140. For example, the control module 435 usesthe renderer 410 to render object views (synthetic images) at the localminima that were found in step 130. In one embodiment, 200 object viewsare rendered. The control module 435 also computes the orientationhistograms of the rendered views. The orientation histograms will beused as the object features (i.e., the basis for classification).

In step 150, the newly-rendered images are added to the training set. Inone embodiment, the control module 435 performs step 150. For example,the control module 435 adds the newly-rendered images (generated in step140) to the training set 450 by storing the images appropriately. Thisgenerates a modified training set 450 that includes both the images thatwere rendered in step 140 and the images that were used to train theclassifier in step 120.

In step 160, the procedure is repeated starting from step 120, so thatthe classifier 445 is retrained using the modified training set 450.

Although the invention has been described in considerable detail withreference to certain embodiments thereof, other embodiments are possibleas will be understood to those skilled in the art. For example, anotherembodiment is described in “Object Recognition with 3D Models” by B.Heisele, G. Kim, and A. Meyer, Proceedings of the 2009 British MachineVision Conference (BMVC), London, England, September 7-10, which ishereby incorporated by reference.

What is claimed is:
 1. A computer-implemented method for training aview-based object recognition classifier, comprising: generating aninitial training set of images, wherein an image in the initial trainingset is generated based on a three-dimensional model and a set ofrendering parameter values, the rendering parameter values comprising atleast one of an azimuth location of a viewpoint, an elevation locationof a viewpoint, and a rotation of a viewpoint around its optical axis;training the classifier using the initial training set; determining theclassifier's accuracy; determining a set of one or more local minima ofthe classifier's output; for each local minimum in the set of localminima: determining a set of rendering parameter values associated withthe local minimum, the rendering parameter values comprising at leastone of an azimuth location of a viewpoint, an elevation location of aviewpoint, and a rotation of a viewpoint around its optical axis; andgenerating an additional image based on the three-dimensional model andthe determined set of rendering parameter values; training theclassifier using the initial training set and the generated additionalimages.
 2. The method of claim 1, wherein the classifier comprises astatistical classifier.
 3. The method of claim 1, wherein the classifiercomprises a support vector machine.
 4. The method of claim 1, whereinthe classifier classifies an image based on a feature vector of theimage's gradient histograms.
 5. The method of claim 1, wherein arendering parameter additionally comprises one element of a groupcontaining: an azimuth location of a point light source, an elevationlocation of a point light source, and an intensity ratio between anambient light and a point light source.
 6. The method of claim 1,wherein a value for a rendering parameter for an image in the initialtraining set is determined randomly or pseudo-randomly.
 7. The method ofclaim 1, wherein an image in the initial training set is associated witha label that specifies whether the image is a positive example or anegative example.
 8. The method of claim 1, wherein determining the setof one or more local minima of the classifier's output comprisesperforming an optimization algorithm.
 9. A non-transitorymachine-readable storage medium encoded with instructions that, whenexecuted by a processor, cause the processor to perform a method fortraining a view-based object recognition classifier, comprising:generating an initial training set of images, wherein an image in theinitial training set is generated based on a three-dimensional model anda set of rendering parameter values, the rendering parameter valuescomprising at least one of an azimuth location of a viewpoint, anelevation location of a viewpoint, and a rotation of a viewpoint aroundits optical axis; training the classifier using the initial trainingset; determining the classifier's accuracy; determining a set of one ormore local minima of the classifier's output; for each local minimum inthe set of local minima: determining a set of rendering parameter valuesassociated with the local minimum, the rendering parameter valuescomprising at least one of an azimuth location of a viewpoint, anelevation location of a viewpoint, and a rotation of a viewpoint aroundits optical axis; and generating an additional image based on thethree-dimensional model and the determined set of rendering parametervalues; training the classifier using the initial training set and thegenerated additional images.
 10. The medium of claim 9, wherein theclassifier comprises a statistical classifier.
 11. The medium of claim9, wherein the classifier comprises a support vector machine.
 12. Themedium of claim 9, wherein the classifier classifies an image based on afeature vector of the image's gradient histograms.
 13. The medium ofclaim 9, wherein a rendering parameter comprises one element of a groupcontaining: an azimuth location of a point light source, an elevationlocation of a point light source, and an intensity ratio between anambient light and a point light source.
 14. The medium of claim 9,wherein a value for a rendering parameter for an image in the initialtraining set is determined randomly or pseudo-randomly.
 15. The mediumof claim 9, wherein an image in the initial training set is associatedwith a label that specifies whether the image is a positive example or anegative example.
 16. The medium of claim 9, wherein determining the setof one or more local minima of the classifier's output comprisesperforming an optimization algorithm.
 17. A system for training aview-based object recognition classifier, comprising: a machine-readablestorage medium encoded with machine-readable instructions for performinga method, the method comprising: generating an initial training set ofimages, wherein an image in the initial training set is generated basedon a three-dimensional model and a set of rendering parameter values,the rendering parameter values comprising at least one of an azimuthlocation of a viewpoint, an elevation location of a viewpoint, and arotation of a viewpoint around its optical axis; training the classifierusing the initial training set; determining the classifier's accuracy;determining a set of one or more local minima of the classifier'soutput; for each local minimum in the set of local minima: determining aset of rendering parameter values, the rendering parameter valuescomprising at least one of an azimuth location of a viewpoint, anelevation location of a viewpoint, and a rotation of a viewpoint aroundits optical axis; and generating an additional image based on thethree-dimensional model and the determined set of rendering parametervalues; training the classifier using the initial training set and thegenerated additional images; and a processor configured to execute themachine-readable instructions encoded on the machine-readable storagemedium.
 18. The system of claim 17, wherein a rendering parameteradditionally comprises one element of a group containing: an azimuthlocation of a point light source, an elevation location of a point lightsource, and an intensity ratio between an ambient light and a pointlight source.
 19. The system of claim 17, wherein a value for arendering parameter for an image in the initial training set isdetermined randomly or pseudo-randomly.
 20. The system of claim 17,wherein an image in the initial training set is associated with a labelthat specifies whether the image is a positive example or a negativeexample.